The law with regard to magic mushrooms is complicated. The psilocybe mushroom or "magic mushroom" is not controlled under the Misuse of Drugs Act 1971, but it contains psilocin, a hallucinogen, and psilocybin, an ester of psilocin, which are controlled as Class A drugs.
There have been a number of court cases concerning the status of psilocybe mushrooms. The courts have held that a person is not in possession of a controlled drug solely by reason of his being in possession of a naturally occurring substance - the mushroom - containing that drug. The growing of psilocybe mushrooms and the gathering and possession of them do not contravene the Misuse of Drugs Act 1971.
It is, however, an offence under the 1971 Act to possess a "preparation" or "product" of the controlled drugs psilocin or psilocybin. The courts have held that where the mushrooms have been prepared (eg by drying or by making into a powder) so that "they have ceased to be in their natural state and have been in some way altered by the hand of man" they constitute a "preparation" or "product" of the Class A controlled drug psilocin. The courts have also held that psilocybe mushrooms that had been frozen and packaged constituted a product containing the Class A controlled drug psilocin.